My wife and I both have T-Mobile Mt3G phones. Intermittently the phone app will use the default ringtome instead of the ringtone defined for my contacts. I have unique ringtones setup for many of my contacts so that I know wehn they call me. If I hear the default ringtone i likely will ignore the call until later. I am not sure if this is android or the phone itself
This happens to me, too. I've noticed that if I have a ringtone for a specific person, it does that correctly. However, if I have a regular contact in my phone that does not have a specific ringtone, sometimes it uses the phone's default ringer instead of the default ringer I have set to use.